(1) Field of the Invention
The present invention relates to a wire or needle printing head comprising a plurality of needles and a corresponding plurality of electromagnets or solenoids. Each solenoid has a fixed core and a movable aramature and the fixed core has two terminal surfaces and can be excited to generate a magnetic flux through these surfaces. The movable armature is of a generally elongate shape and has a first oscillating end for selectively actuating in a printing operation a corresponding needle and a magnetic flux-closure surface in juxtaposed relationship with a first one of the two terminal surfaces. The magnetic flux-closure surface together with the first surface of the fixed core form a main air gap of the electromagnet which is variable with the oscillating movement of the first end of the movable armature.
(2) Description of the Prior Art
Printing heads of this type are used generally in present-day needle printers; the main requirement of such heads is that of providing a high speed of printing with moderate powers of excitation of the solenoids. To achieve that aim it is necessary for the movable parts to be light and for the air gap between cores and movable armatures to be very limited. That is in conflict with the need to have a level of printing energy sufficient for printing a plurality of copies and a relatively long travel movement on the part of the needles.